1. Field of the Invention
The invention relates generally to the field of marine seismic surveying apparatus. More specifically, the invention relates to apparatus for cleaning marine growth and other contaminants from seismic sensor streamers.
2. Background Art
Marine seismic surveying includes having a seismic vessel tow one or more seismic sensor streamers near the surface of a body of water, such as the ocean. Sensor streamers are essentially cables that include a plurality of seismic sensors (usually hydrophones) disposed along the cable at spaced apart locations. The sensors may be protected from water intrusion by enclosing the cable and the sensors in a water tight, acoustically transparent jacket. The interior of the jacket is typically filled with oil, or electrically non-conductive, viscous gel-like materials, both to enable good acoustic coupling between the water and the hydrophones, and to provide electrical insulation between various electrical components, including the hydrophones, in the streamer.
Typical seismic streamers can extend for a substantial distance behind the towing vessel, often several kilometers. As is the case for many devices operated in a body of water, various marine life forms, such as barnacles and seaweed, can become attached to the exterior of seismic streamers. Other contaminants may also become attached to the exterior of the streamer as well. The presence of such contaminants on a streamer, in addition to causing possible deterioration of the streamer, can cause changes in the response of the various sensors in the streamer because the life forms can interfere with transmission of seismic energy through the jacket and to the sensors therein. Such marine growth and other contaminants may cause noise in detected seismic signals and increase the drag. Cleaning such contaminants from the exterior of a streamer is therefore important.
Streamers can be cleaned of such attached marine growth and other contaminants using conventional methods such as brushing and scraping. Such conventional cleaning methods typically require the streamer to be retrieved from the water. Retrieval typically includes spooling or winding the streamer on a suitable winch or similar device. The streamer must then be transported to a facility where it may be unspooled and then cleaned conventionally. Such conventional cleaning procedures can be difficult and expensive to perform, and may require that the streamer be removed from service for a considerable period of time. Alternatively, a streamer can be scraped or cleaned manually from a boat moved alongside the streamer while the streamer is deployed in the water. Such cleaning operations can be difficult to perform, dangerous to personnel in the event of unexpected rough water, and, depending on the skill of the cleaning operator, may risk damage to the streamer.
One type of automatic streamer cleaning device is described in International Patent Application Publication No. WO 2004/051316 (“WO '316”). The streamer cleaning device described in WO '316 includes a housing configured to be affixed to the exterior of the streamer, a vane or similar device affixed to the housing causes the housing to move longitudinally along the streamer as the streamer is towed in the water. A friction drive, consisting of a series of wheels placed in contact with the outer surface of the streamer, converts the longitudinal motion of the housing to rotational energy to operate rotating brushes placed in contact with the streamer.
Another type of automatic streamer cleaning device is described in U.S. patent application Ser. No. 11/227,795 filed on Sep. 15, 2005 and assigned to the assignee of the present invention. The streamer cleaning device disclosed in the '795 application includes a housing that is configured to be removably affixed to the exterior surface of the streamer. The housing includes at least one vane arranged to convert motion of the water therepast, as the streamer is towed into the water, into longitudinal force and rotational force on the housing. Such forces cause the housing to move longitudinally and rotationally about the entire exterior of the streamer. At least one cleaning element, such as a brush or scraper, is disposed inside the housing. The imparted motion of the housing causes corresponding motion of the cleaning element, thus cleaning the streamer.
The cleaning devices described in both WO '316 and in the '795 application are only able to move toward the back end of the streamer using energy generated by movement of water past the cleaning device as the streamer is towed through the water. In order to use either of the forgoing cleaning devices more than once on a streamer, it is necessary to remove the cleaning device from the aft end of the streamer, and reaffix the cleaning device at or near the front end of the streamer.